Oils and fats, also called lipids, are one of the three basic types of calorie sources, the others being carbohydrates and proteins. With 9 calories per gram (38 kJ/g), lipids have the highest food energy content of the three. The other two have 4 calories per gram.

In general, the difference between oils and fats is that, at normal room temperature, fats are solid while oils are liquid. Palm, palm kernel, and coconut oils are exceptions to this rule.

Contents

Many lipids are absolutely essential for life. However, abnormally high levels of certain lipids, particularly trans fatty acids (such as elaidic acid, shown to the right,) are risk factors for heart disease among other diseases, and so should be limited in one's diet.

Humans have a dietary requirement for certain essential fatty acids, such as linoleic acid (an omega-6 fatty acid) and alpha-linolenic acid (an omega-3 fatty acid) because these fats cannot be synthesized from simpler components in the diet. Most vegetable oils (including safflower, sunflower, and corn oils) are rich in linoleic acid. Alpha-linolenic acid is found in the green leaves of plants, and in selected seeds, nuts and legumes (such as flax, canola, walnuts and soy). Fish oils are particularly rich in the omega-3 fatty acids eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA).

A minimum amount of dietary fat is necessary to facilitate absorption of fat-soluble vitamins (A, D, E and K) and carotenoids.

It is claimed by some doctors and nutritionists that a high level of total fat intake contributes to increased risk of obesity and atherosclerosis (the primary cause of coronary and cardiovascular diseases, and the leading cause of illness and death in North America). Others claim that only saturated fats have this effect. Saturated fats have a hypercholesterolemic effect, whereby blood cholesterol levels of both HDL and LDL increase. Many other doctors and nutritionists blame other factors for heart disease.

Intake of monounsaturated fats in oils is thought to be preferable to consumption of polyunsaturated fats in oils because the monounsaturated fats apparently do not lower levels of HDL cholesterol (the "good" cholestrol, versus its "bad" counterpart: LDL). Oxidized LDL is also much more harmful than unoxidized LDL. According to most doctors and nutritionists, keeping cholesterol in the normal range not only helps prevent heart attacks and strokes but may also prevent the progression of atherosclerosis.

Trans. The majority of trans fats consumed are created by the processed food industry. As such, these fats can be found in many fast foods, snack foods, fried foods and baked goods made in (generally larger) facilities. Margarine and shortening may also contain high levels of trans fats, depending on their country of origin- for example, Australian margarine is produced by a different method to American margarine and contains no trans-fat. Naturally occurring: the trans fat in the milk and body fat of ruminants (such as cattle and sheep) is about 2–5% of the total fat. Trans fats are very unhealthy for people.